This invention relates to a sensor which may be used to measure the current flowing in a conductor. More particularly, but not exclusively, this invention relates to a current sensor which may form part of an alternating current (AC) fiscal electricity meter for measuring the energy supplied by a utility company to a business or residential premises.
A variety of different methods are already known for measuring current. One of these methods uses a resistor in the current path to establish a voltage across the resistor which is proportional to the current flowing through the resistor. However, this method suffers from ohmic heating of the shunt resistor and the lack of galvanic isolation between the current being measured and the voltage established.
Other methods of current measurement detect and measure the magnetic field produced by a current flowing through a conductor. These methods generally have the advantage of providing electrical isolation between the current being measured and the circuit performing the measurement. The magnetic field may be detected by a Hall sensor or by a current transformer. The use of a Hall sensor suffers from the disadvantage that Hall sensors can suffer from temperature dependence and are also relatively expensive.
U.S. Pat. No. 5,521,572 discloses an air coupled current transformer which uses two ferromagnetic pole pieces separated by a pair of extensive air gaps. However, this current transformer is not sufficiently accurate for some applications, such as fixed electricity metering, due to its sensitivity to unwanted, externally generated, magnetic field gradients. U.S. Pat. No. 5,736,846 discloses an air coupled current transformer for monitoring the load currents of an audio amplifier. However, this current transformer is also sensitive to magnetic field gradients and therefore is not sufficiently accurate for use in a fiscal electricity meter.
Therefore, both of these examples of prior art current transformer sensors are sensitive to extraneous non-uniform magnetic fields (i.e. where the field strength varies spatially) which may be caused by currents, of the same frequency as the sensed current, flowing through wires which are located in proximity to the current sensor.
Where the current sensor is used as part of a fiscal electricity meter it is particularly important that the meter is not unduly sensitive to the influence of extraneous magnetic fields. There are various published standards relating to the performance of fiscal electricity meters, for example ANSI C12-1993, xe2x80x9cElectricity Meteringxe2x80x9d and IEC1036 second edition 1996-09. More specifically, ANSI C12.16, xe2x80x9cSolid State Electricity Metersxe2x80x9d, Section 10.2.4 xe2x80x9cEffect of External Magnetic Field Test No. 16xe2x80x9d, specifies the degree to which a fiscal electricity meter may be influenced by magnetic interference. The test specifies that with a current of 3 A flowing through the meter, and with the meter placed in one of three specified positions within a 1.8 meter by 1.8 meter loop, that a 100 ampere (100 A) current (of the same frequency and phase as the metered current) flowing around the loop should not alter the meter reading by more than 1%.
Accordingly there is a need for a current sensor which is low in cost, provides isolation from the current being measured, is suitable for integration with modern electronic manufacturing methods and is substantially insensitive to:
(i) magnetic fields as produced, for example, by distant (far field) magnetic sources;
(ii) non-uniform magnetic fields as produced, for example, by large currents flowing through nearby conductors (i.e. field gradients); and
(iii) magnetic fields as specified by the ANSI standard.
According to one aspect, the present invention provides a substantially planar current sensor for establishing an electromotive force proportional to the rate of change of current in a load conductor, the current sensor comprising coil portions where the electromotive force established by the coil portions is substantially equal and opposite for uniform fields from far away interfering sources and is dissimilar for local fields.
Such a sensor may be manufactured as a printed circuit board (PCB) which provides a low cost manufacturing method that can achieve excellent tolerances and hence good reproducibility on the positioning of the conductors that form the coil portions.
According to a second aspect, the present invention provides an electricity meter comprising: an inlet for receiving supply current from an electricity supply; an outlet for outputting the supply current to a load; a primary conductor connected between the inlet and the outlet for providing a current path for the supply current through the meter; a current sensor for sensing and for outputting a measure of the supply current flowing through the primary conductor; means for providing a measure of the voltage of the electricity supply; and means for determining and outputting an indication of energy consumed by the load in dependence upon the current measure and the voltage measure; characterised in that said current sensor comprises a sensing coil having: (a) an inner sensing coil portion having a plurality of conductive turns located in substantially the same plane and having a magnetic centre defined by the configuration of the conductive turns; and (b) an outer sensing coil portion having at least one conductive turn located substantially in the same plane as the conductive turns of the inner sensing coil portion and having a magnetic centre defined by the configuration of the at least one conductive turn; wherein the inner and outer sensing coil portions have substantially equal turns area products, are connected in series and are arranged so that (i) EMFs induced in the inner and outer sensing coil portions by a common background alternating magnetic field oppose each other; and so that (ii) their effective magnetic centres are substantially co-located; wherein said primary conductor has a loop portion which is located adjacent one of said inner and outer sensing coil portions and which lies in a plane substantially parallel to the plane in which the sensing coil portions lie; and wherein the arrangement of the primary conductor and said sensing coil is such that in response to a current flowing in the primary conductor, a signal is induced in said sensing coil which varies in dependence upon the current flowing in the primary conductor.